It should come as no surprise that Russians and Ukrainians are skirmishing in cyberspace. (The term "cyberwar" is probably too strong for what seems to be happening at present, but there could easily be some exploits lurking that, if unleashed, would justify the term "cyberwar.") The Bits blog on the New York Times website reports that distributed denial of service (DDoS) attacks against news organizations in Ukraine have escalated.
In addition to knocking various services offline, hackers today managed for a time to replace the word "Russian" with "Nazi" on the Russia Today website. The Sydney Morning Herald reports that cell phones of Ukrainian members of parliament have been blocked. Also, Ukraine's main government website--kmu.gov.ua--went dark after Russian troops entered Crimea, but experts are unsure whether or not a cyberattack was responsible.
The Russian government has disavowed all responsibility for cyberattacks in Ukraine, just as it did in Estonia and Georgia. In those earlier episodes, officials blamed "patriotic hackers."